Process for producing magnesium metal by electrolysis



Patented Sept. 6, 1932 UNITED STATES PATENT OFFICE GUSTAV PISTOR, OF LEIPZIG, 'AND ROBERT SUCHY, KARL STAIIB, AND WILHEIM MOSCHEL, OF BITTERFELD, GERMANY, ASSIGNORS, BY MESNE ASSIGNMENTS, TO MAGNESIUM DEVELOPMENT CORPORATION, A CORPORATION OF DELAWARE PROCESS FOR PRODUCING MAGNESIUM METAL BY ELECTROLYSIS Application fll'ed November 1929, Serial 1T0. 403,982, and in Germany February 6, 1929.

a The present invention relates to a process for producing magnesium metal by electrolysis of molten chlorides.

An object of the invention is toprovide 5 for an apparatus which, when utilized for said purpose, allows one to obtain a high yield of metal and a chlorine gasof strong concentration. However, various other objects of the invention will appear from the following description without being specifically pointed out in this place.

The electrolytic production of magnesium from magnesium chloride or from a melt containing magnesium chloride-may be carried out in two fundamentally difl'er'ent ways depending on the specific gravity of the electrolyte which may be either lighter or heavier than the magnesium metal to be separated at the cathode. the advantage that the magnesium metal which, in this case, collects on the surface of the electrolyte is entirely free from impurities. At the same time, however, considerable difiiculty arises in the quantitative separation of ,the two products of electrolysis from each other, namely, the chlorine evolved at the anode and the magnesium metal produced at the. cathode. This difiiculty is aggravated when the electrolyzing current is simultaneously used for heating the bath-the latter in this case being provided with asuitable inner lining of insulating'materialbecause on account of the increased current load of the bath comparativelv larger quantities of chlorine are evolved and have to be prevented from entering the catholyte.

These electrolytic products may be separated by means of a diaphragm, which is permeable to the electric current, so as to completely'divide the bath into two separate compartments. It has, however, been found that the structural coherence of all diaphragms, irrespective of the material employed, is destroyed within a-short time by f'the intense overheating caused 1 by the increase in resistance; thus the use of such diaphragms in practice cannot seriously be considered. A partition consisting of a mere wire-net screen, as is, for instance, frequent- The latter alternative ofi'ers ly used in the electrolysis of sodium chloride,

is likewise exposed to destruction within a short time.

Another (method of separatingthe products of electrolysis consists in placing a solid partition of non-porous, non-conducting material between the electrodes which complete- 1y subdivides the bath to at least the depth of the electrodes themselves, both partition and electrodes extendin downwards into the cell. This also results in an efiicient separation of the electrolytic products. However,

as the entire current is thus caused to pass below the lower edge of the partition, the resistance of the bath is increased and, incidentally, a high voltage between corresponding points on both sides of the partition en sues. Now all materials that can be used for such an insulation partition, as, for instance, fireproof ceramic materials, possess a certain, though sometimes but low, degree of porosity and, therefore, in course of time become thoroughly soaked with the electrolyte. The

partition thus becomes conducting; and, owing to the high voltage drop across it, a comparatively large amount of current passes through it instead of below it, thus defeating the purpose of the partition and rapidly destroying it. The partition is most likely to be attacked at the point where it dips into the fused electrolyte. When the'separated cathodically polarized magnesium comes into more or less close contact with the partition, i

beyond the depth of immersion of the partition in the electrolyte. We have furthermore found that the destruction of the partition can be almost completely prevented by immersing both electrodes into the electrolyte to such a depth that they all but touch the bottom of the bath, and so far beyond. the lower edge of the partition that the distance between the partition and the bottom of the bath amounts to more than double the distance from the lower edge of the electrodes to the bottom of the bath. In this case the resistance of the free electrolyte below the partition is, owing to the increase of its conductive area, so much smaller than the resistance of the electrolyte taken up by the pores of the partition that the portion of the current passing through the partition becomes negligible. At the same time also the voltage at corresponding points of both sides of the partition is reduced, and this also tends to diminish corrosion due to thermal or electrolytic action, which eventually ceases to take place entirely.-

It seems surprising that, when proceeding along the lines described above, the buoyancy of the chlorine discharged-atthe anode in the electrolyte, the specific gravity of which is much higher, is suflicient to cause the chlorine bubbles to rise to the surface nearly vertically along the anode.

Inorder to completely collect the chlorine, it is necessary to place the partition at a distance from the anode which increases with v the depth at which the anode is immersed in the bath. When electrolyzing a charge containing molten anhydrous magnesium chloride which is free particularly from carbon particles and silicic acid, the range of scattering of the ascending chlorine bubbles, owing to the viscosity of the electrolyte, is so small even at the surface that said distance need only be a small fraction of the depth to which the electrodes are immersed into the bath.

We have further found that the globules of metallic magnesium separated at the cathode also rise vertically to the surface of the bath in spite of the small difference between their sIpe'cific gravity and that of the' therefore, possible to place electrolyte. t is, the cathode very close to the partition and if, by suitably tapering the lower part of the partition, a certain guidance is given to the globules towards the'cathode compartment, the cathode may beevenplaced beneath .the partition. On the other hand, the partition need only be immersed in the electrolyte to such a depth that the chlorine gas is efliciently prevented from escaping into the cathode i r- By an-arra gement of the partition between the electrodes according to the presin the bath thus permitting the employment of a higher amperage and, therefore, a more economical utilization of the cell.

The arrangement described above is'not limited to cells of particular shape. lVhen employing a circular cell, the cathode and the partition preferably have the shape of concentric cylinders, surrounding the rod-shaped anode placed in the'centre. By placing a suitable lid or cover of non-conductive material upon the cylindrical partition in a gastight manner, providing a gas outlet in the lid and introducing the anode through said lid into the cell, an anode gas chamber maybe may also, in the case of a rectangular cell,

be easily provided by placing, in a gas-tight manner, a lid of refractory material on the rim of the partition-which for this purpose is flush with the rims of the side Walls of the ccll-and the three side walls of the cell surrounding the anode, and providing the ch amber or cupola thus formed with a gas outlet.

Obviously, it is also possiblerto construct double cell units in this manner by employing a square or rectangular cell and by placing cathodes on both sides of the anode and interposing a partition between each cathode and anode. Inthis way a gas chamber for receiving the chlorine evolved may be formed above the anode in a very simple manner, by placing, over the spacebetween the partitions and the adjacent cell walls, a cover, preferably of the same materialas employed for the partitions, and providing said cover with a gasoutlet. Moreover, by combining a plurality of cell elements in series so as to form a larger unit, it is'possible to carry out constructions of electrolytic double cells accordingto the present invention.

Fig. 1 is a vertical section of a cell and Fig. 2 a ground plan of the same cell. Fig. 3 is a vertical section of alsofiiewhat modified cell and Fig. 4 its respective ground plan. Theiron vessel (1 which is provided with a refractory lining b-surro unds the trough c of rectangular cross-section destinedto receive the electrolyte. IA lid d, the two sides of which rest upon the rim of the receivingvessel, is provided on its outer longitudinal sides with the twopartitions e which are fitted in the wall of the receiving vessel and the lower ends of which are at a great distance from the bottom of the cell. In Figs. 1 and 2 the partitions are straight and the oathodes f and f; are placed close to the partition. In Figures 3 and 4 the partitions are provided withextensionstapering towardsthe anode and the cathodes f and f are placed beneath the extension. The anode g is fitted in the lid in a gas-tight manner and the ascending chlorine is collected in the anode chamber thus formed and is led offthrough an outlet 72. so as to be further utilized.

By virtue of the quantitative separation of the products of electrolysis and the low voltages resulting from the condensed arrangement of the electrodes, as described in the present invention, besides obtaining a metal yield of 90% and more, chlorine gas of 97-98% concentration may be recovered.

At the same time the application of parti-- tions as described permits the construction and employment of economical cell units on. a large technical scale without requiring the use of complicated and delicate structures which are never capable of eificiently resisting the mechanical or thermal influences arising in the process.

We claim:

1. An electrolytic cell for producing magnesium from a fused bat-h containing magnesium chloride, comprising a container of non-conductive material and arranged in said container a solid non-conductive substantially vertical partition and vertically disposed electrodes arranged bilaterally to said partition, said partitionand electrodes extending downwards into'said cell, the distance between the lower edge-of said partition and the bottom of the cell being more than twice the distance between either of said electrodes and said bottom.

2. An electrolytic cell for producing magnesium from a fused bath conta ning magnesium chloride, comprising a container of non-conductive material and arranged in said container a solid non-conductive substantially vertical partition and vertically disposed nesium chloride, comprising a container of non-conductive material and arranged in said container 9. solid non-conductiye substantially vertical partition and vertically disposed electrodes arranged bilaterally to said partition, said partition and electrodes extending downwards into said cell, the distance be .tween the lower edge of said partition and smaller than one half of the depth of immersion, in the electrolyte, of either of the electrodes, and the cathode being closer to the partition than the anode.

4. An electrolytic cell for producing magnesium from a fused bath containing magnesium chloride, comprising a container of non-conductive material and arranged in said container a solid, non-conductive partition having a vertically arranged upper part and a lower part tapering towards the anode, a vertical anode on one side of said partition and a vertical cathode placed below said tapering, the distance between the lower edge of said partition and the bottom of the cell being more than twice the distance between either of the electrodes and said bottom, and the distance between said anode and said partitio'n being smaller than one half of the depth of immersion, in the electrolyte, of

.either of said electrodes.

5. An electrolytic cell for producing magnesium from a fused bath containing magnesium chloride, comprising a container of non-conductive material and arranged in said container 9. solid non-conductive substantially vertical partition, an anode and a cathode, both vertically disposed and arranged bilaterally to said partition, said partition and said electrodes extending downwards into said cell, a cover contacting, in a gastight manner, at least with the upper rim of said partition and, forming an anode gas chamber, means for withdrawing a gas from said chamber, and means for passing the anode in a gas-tight manner through said cover,

the distancebetween the lower edge of said- 6. An electrolytic cell unit for producing magnesium from a fused bath containing magnesium chloride comprising a container of non-conductive material and arranged in said container a vertically disposed anode, two substantially vertical partitions of solid non-conductive material arranged bilaterally to said anode and extending wholly across said rectangular cell, and twovertically disposed cathodes arranged on both the outer sides of said partitions, said partitions and electrodes extending downwards into said cell, the distance between the lower edge of said partitions and the bottom ofsaid cell being more than twice the distance between either of said electrodes and said bottom. v.

7. An electrolytic cell unit for producing" l magnesium from a fused bath containin'g means for withdrawing a gas from the cupola thus formed, means for passing said anode:

magnesium chloride comprising a container of non-conductive material and arranged in said container a vertically disposed anode, two substantially vertical flat partitions of solid non-conductive material arranged bilaterally to said anode and extending wholly across said rectangular cell, a cover contacting, in a gas tight manner, with the upper rims of said partitions and the upper rims of the walls of said cell adjacent the points of issue of said partitions from said cell-walls,

through said cupola in a gas tight manner, and two vertically displaced cathodes arranged on both the outer sides of said part-itions, said partitions and electrodes extending downwards into said cell, the distance between the lower edge of said partitions and the bottom of said cell being more than twice the distance between either of said electrodes and said bottom.

8. An electrolytic cell unit for producing magnesium from a fused bath containing magnesium chloride comprising acontainer ofnonconductive material and arranged in said container a vertically disposed anode, two substantia-lly vertical flat'partitions of solid nonconductive material arranged bilaterally to said anode and extending wholly across said rectangular cell, a cover contacting, in a gas tight manner, with the upper rims of said partitions and the upper rims of the walls' of said cell adjacent the points of issue of said partitions from said cell-walls, means for withdrawing a gas from the cupola thus formed, means for passing said anode through said cupola in a gas-tight manner, and two vertically disposed cathodes arranged on both the outer sides of said partitions, said partitions and electrodes extending downwards into said cell, the distance between the lower edge of said partitions and the bottom of said cell being more than twice the distance between either of said electrodes and said bottom, and the distance between the anode and either of said partitions being smaller than one half of the depth of immersion, in the electrolyte, of either of the electrodes.

9. An electrolytic cell unit for producing magnesium from a fused bath containing magnesium chloride comprising a container of non-conductive material and arranged in said container a vertically disposed anode, two solid non-conductive partitions, each having a vertically arranged upper part and a lower part tapering towards the anode, said partitions being arranged bilaterally to said anode and extending wholly across said rectangular cell, two vertical cathodes. placed below -said taperings of said partitions, .a cover contacting, in a gas-tight manner, with the upper rims'of said partitions and' the upper rims of the walls of-said cell adjacent the points of issue of-said partitions from said cell-walls, means for withdrawing a gas 10. An electrolytic apparatus for producing magnesium from a fused bath containing magnesium chloride, comprising a relatively deep electrically insulated container for saidbath, a vertical anode immersed in said bath with its lower end slightly above the bottom of said container, a vertical plate-like cathode spaced from the wall of said container and immersed in said bath" to substantially the same depth as said anode, and a vertical partition extendi across said container between said anode an cathode, and having its lower end submerged a short distance below the surface of said bath, whereby the effective current passes directly from said anode to said cathode without passing through or around said partition.

11. An electrolytic apparatus for producing magnesium from a fused bath containing magnesium chloride, comprising a relatively deep electrically insulated container for said bath, a vertical partition extending across said container intermediate the side walls thereof, and having its lower end submerged a short distance below the surface of said bath, said submerged end being tapered, a

, vertical anode laterally spaced from said partition, and being immersed in saidbath with its lower end slightly above the bottom of said container, a vertical plate-like cathode situated directly beneath, and slightly out of contact with the tapered end of, said partition, and having its lower end substantially in line with the lower end of said anode, whereby the effective current passes directly from said anode to said cathode without pass ing through or around said partition, the taper of the lower end of said partition being toward said .anode to direct the globules of magnesium released at said cathode away from said anode.- 7

' In testimony whereof we have hereunto 

